Video information may be characterized as information representing a succession of frames, with each frame representing a "still" picture which, in turn, is composed of individual picture elements, or pixels. When video information is received by a suitable display device, the frames are displayed in succession, giving an observer the illusion of motion. Common television monitors, for example, receive such video information for display.
Traditionally, video information has been transmitted, received and processed for display in an analog form. However, with the evolution of powerful digital computers having memories and storage devices, such as disks and the like, capable of storing large quantities of digital information, it has become desirable to store video information in a digital form. Typically, when analog video information is converted to a digital form, it is done in such a way that each pixel of each frame is analyzed, and a digital value is created representing the intensity of each such pixel. In the case of color video information, a separate digital value is created representing the intensity of each separate color component of the pixel. For example, each pixel of color video information may be represented by three 8-bit bytes, with one byte (8 bits) representing the intensity of the red component of the pixel, a second byte representing the intensity of the green component of the pixel and a third byte representing the intensity of the blue component of the pixel.
Video information in such a digital form has been stored or otherwise processed successfully. However, because high resolution, full motion, video information often has on the order of tens of thousands of pixels per frame and a frame rate on the order of ten to thirty or more frames per second, storing such video information as described above requires an inordinate amount of digital storage capacity. To reduce the required storage capacity, various methods of compressing the digital information have been attempted. For example, rather than storing full pixel information for each pixel of each frame, only information for those pixels that have changed from the previous frame is stored. Upon retrieving such compressed information, it must be processed (or decompressed) before it can be displayed.
While such a compression technique tends to reduce the required storage capacity, it is most effective where the video information to be displayed is computer generated. For example, the pixels comprising the "blue sky" appearing in the background of a scene which lasts for many frames can be made the same shade of blue for all relevant frames when the information is computer generated. As such, there would be no change at all from frame to frame for those pixels, thereby permitting a high degree of compression.
Where, on the other hand, the scene is filmed live or recorded live on videotape, and then digitized, compression is much less effective, because of noise or color drift introduced by the camera or the analog recording media or the digitization equipment. Where noise or color drift occurs from one pixel of one frame to a corresponding pixel of the next frame, that then becomes a pixel that cannot be compressed. Of course, the resolution of the digital information can be sacrificed in order to increase compression, but that results in a corresponding degradation of the quality of the video display.